The present invention relates to a process for producing a pitch which is substantially soluble in quinoline and is a raw material for producing carbon materials. More in detail, the present invention relates to a process for producing a substantially quinoline-soluble and optically anisotropic pitch for carbon materials, which is suitable as a raw material for various carbon materials necessitating high strength, such as high-performance carbon fibers, graphite fibers or reinforcing materials for composite resin, the optically anisotropic and quinoline-soluble pitch being produced from naphthalene as the raw material and a pitch for carbon materials produced by the process.
The carbon fibers now in market are classified based on raw materials therefor into polyacrylonitrile (PAN)-based carbon fiber produced from PAN and pitch-based carbon fibers produced from pitches. In general, PAN-based carbon fibers have the excellent specific properties, particularly in the point of tensile strength as compared to pitch-based carbon fibers. Accordingly, as the high-performance carbon fibers of high strength and high modulus of elasticity, PAN-based carbon fibers have been in the main current hitherto. However, since the raw material for PAN-based carbon fibers is expensive and the yield of carbonization of PAN is poor, the studies for producing pitch-based carbon fibers, which have the same tensile strength and tensile modulus of elasticity as PAN-based carbon fibers, from the pitch occupying the economical superiority, have been carried out, and several methods therefor have been proposed.
For instance, it has been reported that graphite fibers which has an X-ray diffraction pattern characterized by the presence of (112)-cross lattice line, (100)-line and (101)-line, namely has a high three-dimensional structure and has an interlayer spacing (doo2) of not larger than 3.37 .ANG., an apparent layer size (La) of not smaller than 1000 .ANG. and an apparent layer height (Lc) of not smaller than 1000 .ANG. can be produced by a process comprising the steps of (1) heating a petroleum pitch, coal tar pitch or acenaphthylene pitch at 350.degree. to 500.degree. C. for a sufficient time for forming a mesophase of about 40 to 90% by weight, thereby obtaining a carbonaceous pitch which is non-thixotropic and has a viscosity of 10 to 200 poise at the spinning temperature thereof, (2) spinning the thus obtained carbonaceous pitch, thereby obtaining pitch fibers, (3) infusibilizing the thus obtained pitch fibers at 250.degree. to 400.degree. C. in an oxygen-containing atmosphere, thereby obtaining infusibilized fibers, (4) heating the thus obtained infusibilized fibers at a temperature not lower than 1000.degree. C. in an inert atmosphere and then (5) heating the thus heated fibers at a temperature not lower than about 2500.degree. C. (refer to Japanese Patent Application Laid-Open No. 49-19127/1974).
As has been disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 49-19127/1974, it has been considered hitherto that the use of the mesophase pitch is indispensable for producing the high-performance pitch-based carbon fibers. The above-mentioned consideration is due to the fact that in the case of melt-spinning the mesophase pitch which has molecular orientation, the molecules of the pitch are easily aligned parallel to the fiber axis. However, since the softening point of the mesophase pitch is generally high, there is a defect therein that the temperature of melt-spinning thereof is high and the pitch becomes unstable thermally. Further, since the mesophase pitch is a heterogeneous mixture of an isotropic pitch and liquid crystal of pitch, it has been deemed that it is difficult to obtain the homogeneous pitch fibers from the mesophase pitch.
In order to solve the above-mentioned weak points a pitch usable for melt spinning, which is not necessarily optically anisotropic in the stage of the pitch for spinning, however, which is excellent in spinnability and is converted into an optically anisotropic state in the stage of spinning or calcining, and a process for producing carbon fibers by using such a pitch have been proposed.
For instance, Japanese Patent Application Laid-Open No. 58-18421/1983 reports a process comprising the steps of (1) melt-spinning an optically isotropic premesophase carbonaceous substance or a pitch-like substance mainly consisting of an optically isotropic premesophase carbonaceous substance under the melt-spinning conditions of not substantially increasing the amount of mesophase carbonaceous substance, (2) infusibilizing the thus melt-spun fibers and (3) carbonizing the thus infusibilized fibers, thereby converting the premesophase carbonaceous substance or the pitch-like substance containing the premesophase carbonaceous substance into the optically anisotropic mesophase carbonaceous substance substantially.
Further a dormant anisotropic pitch of the atomic ratio of hydrogen to carbon (H/C) of 0.55 to 1.2, which (1) contains as a component for forming the dormant anisotropy, a substantially quinoline-soluble polycyclic and polynuclear hydrocarbon which is obtained by partially hydrogenating polycyclic and polynuclear hydrocarbons existing in mesophase pitch, (2) forms, in the molten state thereof, a wholly homogeneous and optically isotropic single phase without substantially forming mesophase and (3) in the case where an external force is applied thereon, shows a tendency of preferred orientation along the direction of the external force, is disclosed (refer to EP-A2-54,437).
However, in every one of the above-mentioned cases, a separate step such as hydrogenation treatment is necessary and particularly, a specified spinning condition is necessary. Accordingly, it was difficult to obtain the high-performance carbon fiber.
Moreover, in the former case, there was no example of producing carbon fibers while singly using the premesophase pitch, namely the quinoline-soluble pitch, and the pitch for melt-spinning is the material containing a quinoline-insoluble component.
The present inventors have also found out that a conspicuous optical conversion of from isotropy to anisotropy progresses and the carbon fibers or graphite fibers of high performance are obtained in the process comprising the steps of (1) polymerizing naphthalene as the raw material by heating thereof in the presence of a Lewis acid catalyst, (2) removing the catalyst from the reaction mixture, thereby obtaining a pitch-like substance, (3) heating the thus obtained substance under an ordinary atmospheric pressure or a reduced pressure while flowing an inert gas, thereby removing volatile components and obtaining a precursor pitch which shows the low softening point, does not contain polycyclic and polynuclear high molecular component of progressed molecular orientation (which is the quinoline-insoluble component), shows the optical isotropy and is easily subjected to spinning, (4) spinning the thus obtained precursor pitch, thereby obtaining the optically isotropic pitch fibers and (5) infusibilizing, calcining and carbonizing the thus obtained pitch fibers. (Refer to U.S. patent application Ser. No. 773,037 now abandoned, GB-A-2,164,351).
However, in the thus obtained carbon fibers, there was a room for improvement concerning the mechanical specific properties (particularly, concerning the tensile modulus of elasticity).
Generally, in the case of producing the high-performance carbon fibers, it is necessary that the pitch for spinning is provided with the molecular orientability to the extent that the spun fibers can be oriented in the step of spinning and, at the same time, with the spinnability and the fluidity. Hitherto, the molecular orientability of the mesophase pitch containing the quinoline-insoluble component has been raised by increasing the molecular weight by thermal treatment. However, there have been problems therein concerning the spinnability and thermal stability at the time of spinning such a pitch. Further, in the case of melt-spinning the above-mentioned mesophase pitch containing the quinoline-insoluble components, it is apt to form a large leaf-form domain in the pitch fiber due to the high orientability thereof. Accordingly, the temperature of melt-spinning should be raised, the pitch becomes thermally unstable and there is a weak point that the homogeneous pitch fibers are not available.
Namely, in order to avoid the above-mentioned defects, a specified condition of spinning was necessary.
The internal structure and the performance of carbon fibers are largely influenced by the chemical structure of the pitch used for spinning. Hitherto, since the pitch fibers spun from the optically isotropic pitch scarcely had the orientability, the thus obtained pitch fibers are particularly poor in the tensile modulus of elasticity, and in order to obtain the high-performance carbon fibers, it was necessary to suitably select the conditions of the treatment of infusibilization and to carry out the stretching and calcining at a high temperature. Such a process was industrially disadvantageous.
As a result of the present inventor's earnest researches for solving the above-mentioned problems, the following information was obtained and on the basis of the information, the present invention have been completed.
The pitch produced from naphthalene as the raw material comprises the chainly-linked aromatic compounds having 6 to 12 naphthalene rings and naphthene rings. The planarity of the above-mentioned ring component has been distorted due to the presence of the naphthene ring therein.
By subjecting the above-mentioned pitch to a thermal treatment in the first stage of thermal treatment at a temperature of 390.degree. to 450.degree. C. for 1 to 5 hours under a pressure of not less than an atmospheric pressure so as to produce neither a quinoline-insoluble pitch nor optically anisotropic pitch, the polymerization of the unstable substances remaining in the pitch proceeds or the condensation of the naphthalene rings proceeds by partial dehydration from the naphthene rings resulting in the conversion of the pitch into the stable compounds. Namely, the thermal stability of the pitch is raised (the above-mentioned step is referred to as the aromatization reaction). The first stage of thermal treatment is carried out in a closed system or an open system.
The volatile components generated from the pitch in decomposition by the first stage of thermal treatment is removed by carrying out the second stage of thermal treatment at 350.degree. to 380.degree. C. under a reduced pressure in a flow of an inert gas.
Since the thermally stabilized component has the carbon skeleton which can constitute the mesophase, an optical anisotropy becomes observable in the pitch subjected to the second stage of thermal treatment.
As mentioned above, by promoting the aromatization of the pitch as compared to the case of obtaining the conventional quinoline-soluble and optically isotropic pitch and subjecting the pitch to thermal treatment under the conditions by which the component of high molecular weight becoming the quinoline-insoluble material does not form, it was possible to obtain the pitch which shows the suitably low softening point and melt viscosity, is excellent in spinnability, is substantially soluble in quinoline and shows the optical anisotropy.
Further, by using the above-mentioned quinoline-soluble and optically anisotropic pitch, it was possible to obtain the high-performance carbon fibers and graphite fibers which have the anisotropic structure and in which the tensile modulus of elasticity has been further improved as compared with the carbon fibers obtained from the conventional quinoline-soluble and optically isotropic pitch, without relying upon any specified condition of spinning.